Helm's Deep is a huge fortress in J.R.R Tolkien's Lord of the Rings trilogy. It plays a prominent role in the second book in the trilogy, The Two Towers, where it is the site of a battle between Saruman's Uruk-hai and the Rohirrim of King Théoden, assisted by Aragorn, Legolas, and Gimli. The defenders are greatly outnumbered, and gradually lose more and more ground, before getting rescued by timely reinforcements from Gandalf and Erkenbrand. Théoden's heroic charge, leaving the ruined wall of the fortress behind to lead the Rohirrim out to face the Uruk-hai directly, is nothing to be sneezed at, either.
In the movies, the Rohirrim are joined in the Battle of Helm's Deep by a contingent of Elves from Lothlorien led by Haldir. The fact that the Elves had enough to worry about defending their own borders seems not to have deterred the movie-makers from letting Haldir and his people die a rather pointless death, as their presence doesn't really help the Rohirrim at all--Gandalf still has to rescue them. This choice is one that many "Tolkien purists" (some PPCers among them) find to be incredibly stupid.